1. Field of the Invention
The present invention relates to a liquid crystal shutter array used for optical printers and, more specifically, to a color liquid crystal shutter array that makes it possible to effect full-color printing onto photosensitive papers. The invention further relates to a method of driving the liquid crystal shutters in a liquid crystal optical printer which optically writes pictures onto photosensitive papers and photosensitive materials.
2. Description of the Related Art
The liquid crystal shutter array is a device in which a plurality of liquid crystal pixels arranged in a row or in a plurality of rows are turned on and off to control the transmission of light through each of the pixels. A method of full-color printing of a picture onto a photosensitive paper by using the liquid crystal shutter array has been disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 62-134624. According to this method, the full-color picture is formed on a photosensitive paper by utilizing a combination of a rotary plate on which are mounted a white-and-black liquid crystal shutter array and red-color, green-color and blue-color filters, and which is rotated by a motor, and an irradiation device which selectively emits red light, green light and blue light.
However, this method requires a rotary exposure system which separates and selects white light into color light, and an increased number of parts make the color printer bulky. Moreover, the timing for opening and closing the liquid crystal shutter pixels must be brought into agreement with the motion of the photosensitive paper that intermittently changes the rotating period of the rotary exposure device, making it difficult to obtain a full-color picture having good precision.
According to a method disclosed in Japanese Unexamined Utility Model Publication No. 61-125453, on the other hand, red-color, green-color and blue-color filters are provided for each of the rows of pixels of a liquid crystal shutter array that has a plurality of rows of pixels, and the lights that have passed through the filters having different hues are successively projected in an overlapping manner onto a photosensitive paper that moves at a constant speed perpendicularly to the rows of pixels, in order to obtain a picture in full color. This method is considered to be very helpful in reducing the size of a color printer.
According to the liquid crystal shutter array disclosed in the above publication, in which matrix pixels formed by many signal electrodes that intersect the electrodes of three rows are driven in a multiplexed manner, however, the quality of the printed picture is deteriorated due to a decrease in the contrast ratio of the pixels; and an increased period of printing time is required due to a decrease in the response speed, thus deteriorating the characteristics of the color printer.
A driving method which enables the liquid crystal shutter elements to exhibit their characteristics to a maximum degree, in regard to contrast ratio and response speed, can be represented by a static driving method which drives the pixels independently of each other. This method makes it possible to print a color picture of high quality on a photosensitive paper.
However, when it is attempted to apply this static driving method to the color liquid crystal shutter array having rows of red, green and blue pixels, wiring space must be maintained among the red pixels (R) in the color liquid crystal shutter array 41 to run lead electrodes 42 that are drawn from the pixels 44 of green color (G) as shown in FIG. 1. Therefore, the space portions among the neighboring pixels are not exposed by any of the pixels and, hence, the picture that is formed contains a number of stripes in parallel with a direction 43 in which the color liquid crystal shutter array moves, causing the quality of the printed picture to be greatly deteriorated.
On the other hand, the liquid crystal optical printers that have been developed or put into practice thus far, are all monochromatic hard copy printers without gradation to compete with laser printers and LED printers, and importance has been placed chiefly on high-speed performance.
Here, when it is attempted to copy a colored and gradated picture such as a photo picture or a TV picture by using a liquid crystal optical printer, the liquid crystal shutters which are principal constituent elements must have a contrast which is as high as 100 or more and must further have gradation. For this purpose, the amount of light transmitting through each pixel of the liquid crystal shutter per unit time must be controlled depending upon the picture input signal.
A method of solving the above-mentioned problem has been disclosed in Japanese Unexamined Patent Publication No. 62-150330. FIGS. 2(a) to 2(c) are diagrams which explain this method. The liquid crystal shutter to be driven is a positive-type twisted nematic liquid crystal shutter constituted by a liquid crystal cell of twisted nematic liquid crystal sandwiched by a pair of glass substrates with a transparent electrode, which is further sandwiched by a pair of polarizer plates which are so arranged that the axes of absorption intersect at right angles with each other. An AC rectangular liquid crystal drive wave shown in FIG. 2(b) based on a picture signal of FIG. 2(c) is applied to the transparent electrodes of the liquid crystal shutter. As a result, shutter transmission intensity characteristics are obtained as shown in FIG. 2(a). According to this method, the light is usually shut off by applying a voltage, and the application of voltage is interrupted when a picture signal is input permitting the light to pass through, to thereby control the amount of light transmission.
Through experiments, however, the inventors have learned that the above-mentioned driving method is not really capable of offering a sufficient contrast ratio. Usually, the liquid crystal shutter array for printing pictures has nearly 1000 signal electrodes and is generally driven by a CMOS-LSI from the viewpoint of decreasing both the size of the module that is mounted, and the cost. However, CMOS-LSI has a driving voltage of about 15 volts at the greatest. When the 240 degree-twisted positive-type liquid crystal shutters were driven with this voltage, however, the obtained contrast ratio was about 60 degrees though it may vary depending upon the liquid crystal materials. A picture of satisfactory quality was not obtained even when the color picture was written on a Polaroid 669 instant film with the above-mentioned contrast ratio.
Moreover, the ordinary super-twisted nematic liquid crystal element is colored yellow since it produces a display by utilizing the effect of double refraction. Therefore, it becomes very difficult to print a vivid full-color picture by using the above-mentioned liquid crystal elements.